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From: Song Liu <songliubraving@fb.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: bpf <bpf@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"acme@kernel.org" <acme@kernel.org>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"kjain@linux.ibm.com" <kjain@linux.ibm.com>,
	Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH v5 bpf-next 1/3] perf: enable branch record for software events
Date: Tue, 7 Sep 2021 18:59:43 +0000	[thread overview]
Message-ID: <B3CCDCB4-1D06-4331-A3C7-B1D413A4ABA5@fb.com> (raw)
In-Reply-To: <D501C4AD-3778-431B-A710-3399BFE6EE56@fb.com>



> On Sep 3, 2021, at 9:50 AM, Song Liu <songliubraving@fb.com> wrote:
> 
> 
> 
>> On Sep 3, 2021, at 1:02 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>> 
>> On Thu, Sep 02, 2021 at 09:57:04AM -0700, Song Liu wrote:
>>> +static int
>>> +intel_pmu_snapshot_branch_stack(struct perf_branch_entry *entries, unsigned int cnt)
>>> +{
>>> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>>> +
>>> +	intel_pmu_disable_all();
>>> +	intel_pmu_lbr_read();
>>> +	cnt = min_t(unsigned int, cnt, x86_pmu.lbr_nr);
>>> +
>>> +	memcpy(entries, cpuc->lbr_entries, sizeof(struct perf_branch_entry) * cnt);
>>> +	intel_pmu_enable_all(0);
>>> +	return cnt;
>>> +}
>> 
>> Would something like the below help get rid of that memcpy() ?
>> 
>> (compile tested only)
> 
> We can get rid of the memcpy. But we will need an extra "size" or "num_entries" 
> parameter for intel_pmu_lbr_read. I can add this change in the next version. 
> 

This is trickier than I thought. As current lbr_read() function works with 
perf_branch_stack, while the BPF helper side uses array of perf_branch_entry. 
And the array is passed into the helper by the BPF program. Therefore, to 
really get rid of the memcpy, we need to refactor the lbr driver code more. 
How about we keep the memcpy for now, and add the optimization later (if we 
think it is necessary)?

Thanks,
Song


  reply	other threads:[~2021-09-07 18:59 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-02 16:57 [PATCH v5 bpf-next 0/3] bpf: introduce bpf_get_branch_snapshot Song Liu
2021-09-02 16:57 ` [PATCH v5 bpf-next 1/3] perf: enable branch record for software events Song Liu
2021-09-02 20:49   ` John Fastabend
2021-09-03  8:02   ` Peter Zijlstra
2021-09-03 16:50     ` Song Liu
2021-09-07 18:59       ` Song Liu [this message]
2021-09-07 20:50         ` Andrii Nakryiko
2021-09-03  8:27   ` Peter Zijlstra
2021-09-03 16:45     ` Song Liu
2021-09-04 10:18       ` Peter Zijlstra
2021-09-02 16:57 ` [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot Song Liu
2021-09-02 20:56   ` John Fastabend
2021-09-02 22:04     ` Song Liu
2021-09-02 22:53   ` Andrii Nakryiko
2021-09-02 23:03     ` Song Liu
2021-09-02 23:05       ` Andrii Nakryiko
2021-09-03  1:03   ` Alexei Starovoitov
2021-09-03  8:28   ` Peter Zijlstra
2021-09-03 16:58     ` Song Liu
2021-09-03  8:47   ` Peter Zijlstra
2021-09-03 17:06     ` Song Liu
2021-09-03 17:10     ` Andrii Nakryiko
2021-09-04 10:24       ` Peter Zijlstra
2021-09-04 10:55         ` Peter Zijlstra
2021-09-02 16:57 ` [PATCH v5 bpf-next 3/3] selftests/bpf: add test for bpf_get_branch_snapshot Song Liu
2021-09-02 21:05   ` John Fastabend
2021-09-02 22:54 ` [PATCH v5 bpf-next 0/3] bpf: introduce bpf_get_branch_snapshot Andrii Nakryiko

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